Typically, in a print stroke of a screen printing machine, a squeegee will pass over the screen once, pushing the ink through the mesh openings in the screen. Some systems will have a feature permitting a second print stroke, or a second pass over the screen. It is widely believed that this will produce a thicker and smoother image on the textile being printed upon. The second print stroke, if selected, will be at the same pressure as the first stroke. It is well known that squeegee pressure, which translates to the pressure or force the squeegee places upon the screen during the print stroke, affects the amount of ink pushed through the gaps or openings on the screen and onto the textile being printed upon. Accordingly, many falsely believe the more ink deposited upon the textile, the better the results. This is wrong in many instances. Merely adding pressure to the squeegee and/or adding print strokes does not always improve the quality of the finished product.
Rather, it has been found that for several reasons, customizing the pressure of the strokes and the adding of strokes improve the quality of the image. For example, there are many factors contributing to the image created on the textile. As a starter, these include the ink employed and the textile printed upon. Different inks behave differently and different textiles act differently.
Inks include Plastisol (with and without additives, such as expanding inks), water based inks, PVC/Phalate Free, discharge inks (which remove die), foil, glitter/shimmer, metallic, caviar beads, glosses, nylobond, mirrored silver and other solvent based inks. Textiles include natural and artificial fibers from animals (e.g., wool and silk), plants (e.g., cotton, flax, jute, hemp, modal, piña and ramie), minerals (e.g., glass fibers) and synthetics (e.g., polyester, aramid, acrylic, nylon, spandex/polyurethane, olefin, ingeo and lurex). Each combination of ink and textile will demonstrate different properties, such as those associated with wicking, holding, hand, penetration and appearance. Accordingly, a one-size-fits-all approach does not necessarily produce the best results. Specifically, it has been found the appearance of an image will change on a textile with multiple printings at the same or different pressures. For example, performing three print strokes while incrementally increasing the squeegee's pressure on the screen (and hence the textile) will produce different results than performing three print strokes while incrementally decreasing the squeegee's pressure on the screen. With one combination of ink and textiles the multiple strokes increasing may be better and with another combination of ink and textiles the multiple strokes decreasing may be better.
In summary, permitting one to selectively increase the number of print strokes by a squeegee and vary the pressure applied by the squeegee gives one additional options and important tools towards improving the final printed product.
Applicants of the present invention have also recognized the final image on the textile can often be greatly improved if the textile is “kissed” by the screen during the last print stroke by the squeegee. This so-called kissing of the textile is accomplished by using as little pressure on the screen as possible so as to have the textile barely touching the screen when the squeegee passes thereover forcing the ink therethrough. As a result, the last print stroke is at a very low squeegee pressure.
In addition to the above, fibrillation is a common issue in screen printing upon textiles. It generally means “fibers showing through.” It exists when fibers break through the layer of ink laid thereinover. Specifically, instead of the ink totally covering the textile, sporadic fibers will appear on the outermost surface of the ink. This will give the product an unfinished or imperfect look. It will frequently affect the appearance of the print for the color of the textile will appear in the print giving the image a washed-out appearance. Fibrillation may also cause poor resolution when the desired design relies on the textile substrate for very small or fine parts of the design.
It has been found that abrasion of the area print upon can cause the fibers of the textile to break loose from the surface of the ink, giving the print a ‘washed-out’ appearance.
Fibrillation is dependant on many variables, such as the ink weight and viscosity, type of ink, type of textile and the weave of a textile. For example, it is believed that the higher the ink weight the lesser the chance of fibrillation. In addition, the tighter the fibers on the surface of the textile the lesser the chance of fibrillation. Other factors affecting fibrillation include the design of the print, the gaps in the screen and thickness of the screen, viscosity of the ink, and the cure times, also are factors.
One solution is to print the textile, flash it and overprint it with a clear plastisol or water-based clear ink. This has been found to minimize fibrillation. However, this can cause a glossing or mottling effect.
Applicants of the present invention have observed that one cause of fibrillation is the pressure applied by the squeegee during the print stroke. Specifically, when the print stroke is completed, the screen rebounds from the textile. At the same time, both ink and fibers are drawn away from the textile. The result is that at times, fibers can be drawn further from the textile than the ink causing fibers to overlay or rest on top of the outer layer of ink.
Applicants have found that reducing the pressure to the squeegee on the last stroke draws fewer fibers and covers the fibers drawn by prior print strokes. Accordingly, a last kissing stroke can rectify fibrillation or minimize it.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior automated printing machines of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.